This invention relates to the casting of metal strip. It has particular but not exclusive application to the casting of ferrous metal strip.
It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term "nip" is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between end closure side plates or dams held in sliding engagement with the ends of the rolls.
Although twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, there have been problems in applying the technique to the casting of ferrous metals which have high solidification temperatures and a tendancy to produce defects caused by uneven solidification at the chilled casting surfaces of the rolls. When casting ferrous strip it is particularly important to maintain a required metal flow distribution across the width of the casting rolls and defects can occur due to minor flow fluctuations from the required metal flow distribution. It is therefore important to achieve steady state casting conditions with very accurate control over the casting pool level and the casting speed. It has previously been proposed to continuously monitor the casting pool level and to control the flow of metal to the delivery nozzle by operation of a flow control valve in response to the pool level measurements in order to maintain an optimum pool level. An arrangement of this kind is described in our Australian patent 642049 which fully describes the construction and operation of an appropriate metal flow control valve.
Controlling the flow of metal to the delivery nozzle in response to pool level measurements enables accurate control of the pool level during steady state casting conditions. However this form of control is insufficient to deal with the problem of establishing even cooling and solidification on initial start-up when the casting pool is being established and filled to an operational level. It is essential to achieve even cooling and solidification very rapidly in order to allow continuous casting to be initiated before steady state conditions can be established to allow casting to proceed under optimum conditions. To meet these requirements the casting pool must be filled very quickly but in a controlled manner without overshooting a controlled rate of fill so as to enable the metal to solidify and form a coherent strip under start-up conditions.
One possible start-up technique is simply to operate the flow control valve in a predetermined flow control sequence designed to produce a predicted rise in pool level through the start-up period. Specifically, the control valve may be moved in incremental steps from an open condition toward a more restricted condition so that the rate of pool level increase reduces as the level approaches the required operational level. However, the condition of the rolls and the casting pool can change very rapidly during start-up. These fluctuations cannot be accurately forecast and the rising pool level will invariably tend to vary from the predicted and desired start-up pattern. Because of the time delay between changes in the setting of the control valve and consequent effects in the casting pool, it is impossible to control such variation by movement of the control valve in response to actual pool level measurements. The present invention addresses this problem by providing a two-stage start-up procedure. In the first stage, the initial start-up phase, the rise of the pool level during filling of the pool is controlled by varying the rotational speed of the casting rolls in response to instantaneous pool level measurements. Variation of the roll speed variations can produce a very rapid change of pool level and it has been found that it is possible by controlling the speed of the rolls in combination with operation of the control valve in a predetermined sequence to accurately control the rise of the pool level to conform with a required pattern. This initial start-up phase permits the roll speed to depart from the desired optimum speed for steady state casting. In the second stage, the transition phase, any variation of the roll speed from the desired optimum speed is used to cause adjustment of the control valve to enable the roll speed to be brought within a desired speed range. Once within the desired pool level and optimum speed range the invention provides for a steady-state phase of control in which pool level variations are adjusted directly by the control valve and speed is controlled in response to the instantaneous pool level.